Single-site catalysts containing chelating N-oxide ligands

ABSTRACT

A single-site olefin polymerization catalyst and method of making it are disclosed. The catalyst comprises an activator and an organometallic complex. The complex comprises a Group 3 to 10 transition or lanthanide metal, M, and at least one chelating N-oxide ligand that is bonded to M. Molecular modeling results indicate that single-site catalysts based on certain chelating N-oxide ligands (e.g., 2-hydroxypyridine) will rival the performance of catalysts based on cyclopentadienyl and substituted cyclopentadienyl ligands.

FIELD OF THE INVENTION

[0001] The invention relates to catalysts useful for olefinpolymerization. In particular, the invention relates to “single-site”catalysts that incorporate at least one chelating N-oxide ligand.

BACKGROUND OF THE INVENTION

[0002] Interest in single-site (metallocene and non-metallocene)catalysts continues to grow rapidly in the polyolefin industry. Thesecatalysts are more reactive than Ziegler-Natta catalysts, and theyproduce polymers with improved physical properties. The improvedproperties include narrow molecular weight distribution, reduced lowmolecular weight extractables, enhanced incorporation of a-olefincomonomers, lower polymer density, controlled content and distributionof long-chain branching, and modified melt rheology and relaxationcharacteristics.

[0003] Traditional metallocenes commonly include one or morecyclopentadienyl groups, but many other ligands have been used. Puttingsubstituents on the cyclopentadienyl ring, for example, changes thegeometry and electronic character of the active site. Thus, a catalyststructure can be fine-tuned to give polymers with desirable properties.Other known single-site catalysts replace cyclopentadienyl groups withone or more heteroatomic ring ligands such as boraaryl (see, e.g., U.S.Pat. No. 5,554,775), pyrrolyl, indolyl, (U.S. Pat. No. 5,539,124), orazaborolinyl groups (U.S. Pat. No. 5,902,866). Amine oxides are widelyused in the polymer industry as stabilizers (see, for example, U.S. Pat.No. 5,268,114), and many are commercially available. Seldom, however.have amine oxides been used in a process for polymerizing olefins or asa component of an olefin polymerization catalyst. An exception is U.S.Pat. No. 4,015,060, which teaches to use sterically hinderedheterocyclic amine oxides (such as pyridine N-oxide or 2,6-lutidineN-oxide) in combination with a Ziegler-Natta catalyst (titaniumtrichloride, a trialkyl aluminum, and a dialkyl aluminum halide) topolymerize propylene. The amine oxide reduces the amount oflow-molecular-weight, alkane-soluble impurities in the desired product,crystalline polypropylene.

[0004] In contrast, single-site olefin polymerization catalysts thatcontain N-oxide ligands are not known. Also unknown are catalysts thatincorporate a chelating N-oxide ligand, i.e., one that can form achelate using the N-oxide oxygen atom and a second atom that can donatean electron pair to the transition metal.

[0005] The commercial availability of many N-oxides and the ease withwhich a host of other interesting N-oxide ligands can be prepared (e.g.,by simply oxidizing the corresponding tertiary amine with hydrogenperoxide or a peracid) suggests that single-site catalysts withadvantages such as higher activity and better control over polyolefinproperties are within reach. Ideally, these catalysts would avoid theall-too-common, multi-step syntheses from expensive, hard-to-handlestarting materials and reagents.

SUMMARY OF THE INVENTION

[0006] The invention is a single-site olefin polymerization catalyst.The catalyst comprises an activator and an organometallic complex. Theorganometallic complex comprises a Group 3 to 10 transition orlanthanide metal, M, and at least one chelating N-oxide ligand that isbonded to M.

[0007] Evidence from molecular modeling studies suggests thatsingle-site catalysts based on chelating N-oxide ligands (e.g.,2-hydroxypyridine N-oxide) will rival the performance of catalysts basedon cyclopentadienyl and substituted cyclopentadienyl ligands.

[0008] The invention includes a simple synthetic route to thesingle-site olefin polymerization catalysts. The ease and inherentflexibility of the synthesis puts polyolefin makers in charge of a newfamily of single-site catalysts.

DETAILED DESCRIPTION OF THE INVENTION

[0009] Catalysts of the invention comprise an activator and anorganometallic complex. The catalysts are “single site” in nature, i.e.,they are distinct chemical species rather than mixtures of differentspecies. They typically give polyolefins with characteristically narrowmolecular weight distributions (Mw/Mn<3) and good, uniform comonomerincorporation.

[0010] The organometallic complex includes a Group 3 to 10 transition orlanthanide metal, M. More preferred complexes include a Group 4 to 6transition metal; most preferably, the complex contains a Group 4 metalsuch as titanium or zirconium.

[0011] The organometallic complex also comprises at least one chelatingN-oxide ligand that is bonded to the metal. By “chelating,” we mean thatthe ligand can bind to a transition metal using the oxygen atom of theamine oxide and one other atom that can donate an electron pair to themetal. The other atom is preferably separated from the amine oxideoxygen by 2 to 5 atoms. The other electron-donating atom can be neutral(as in a hydroxyl, alkoxy, or amino group) or anionic (as in adeprotonated hydroxyl, deprotonated amine, or carbanion). In preferredchelating N-oxide ligands, the electron-donating atom is anionic. Inother words, the ligand is preferably deprotonated before incorporatingit into the transition metal complex. The electron-donating atom can beoxygen, nitrogen, sulfur, phosphorus, or carbon.

[0012] A preferred class of chelating N-oxide ligands are heterocyclicaromatic amine oxides that have an electron-donating atom ortho to theamine oxide nitrogen. Deprotonation of these ligands generates aresonance-stabilized anion. Ligands in this group include, for example,N-substituted imidazole N-oxides, pyridine N-oxides, and lutidineN-oxides that have an electron-donating group in the ortho position.

[0013] Examples include 2-hydroxy-1-methylimidazole N-oxide,2-hydroxypyridine N-oxide, 2-hydroxyquinoline N-oxide,2-hydroxy-4,6-dimethyllutidine N-oxide, 2-(N′-methylamino)pyridineN-oxide, 2-(2-phenethyl-2-oxo)pyridine N-oxide, and the like. Alsosuitable are heterocyclic amine oxides having an electron-donating atomwithin 3 atoms of the amine oxide nitrogen. An example is8-hydroxyquinoline N-oxide.

[0014] Other aliphatic and cycloaliphatic amine oxides having electrondonor groups are also suitable because of their ability to stabilize thetransition metal in an active single-site complex. Examples areN-hydroxyethyl-N,N-dibutylamine N-oxide,N-hydroxyethyl-N,N-diphenylamine N-oxide,N-methyl(2-hydroxymethyl)piperidine N-oxide,N-methoxyethyl-N,N-dimethylamine N-oxide, and the like. The amine oxidesare conveniently prepared by oxidizing the corresponding tertiary amineswith hydrogen peroxide or a peroxyacid in aqueous or organic mediaaccording to well-known methods. See, for example, U.S. Pat. Nos.5,955,633, 5,710,333, 5,082,940, 4,748,275, 4,504,666, and 4,247,480,the teachings of which are incorporated herein by reference.

[0015] Suitable chelating N-oxides also include those prepared byoxidizing a nitrogen of the corresponding imines (R—N═CR′R″) or azocompounds (R—N═N—R′) that have an electron donor group within 5 atoms ofthe N-oxide oxygen. In the formulas, R and R′ are alkyl, aryl, oraralkyl (preferably both aryl) groups, and R″ is hydrogen or an alkyl,aryl, or aralkyl group. Examples from these groups areazobis(2-hydroxybenzene) N-oxide (I) and benzophenoneN-(2-hydroxyphenyl)imine N-oxide (II):

[0016] More precisely, compound (I) is an “azoxybenzene” (see J. March,Advanced Organic Chemistry, 2^(nd) ed., (1977) p. 1111) and compound(II) is a “nitrone.” Nitrones are conveniently prepared by condensingaldehydes (e.g., benzaldehyde) and hydroxylamines (e.g.,N-phenylhydroxylamine) as shown in Organic Syntheses, Coll. Vol. V, p.1124. Nitrones can also be made by imine oxidation using a peroxyacid(J. Chem. Soc., Perkin Trans. I (1977) 254), by hydrogen peroxideoxidation of secondary amines in the presence of aqueous sodiumtungstate (Org. Synth., Coll. Vol. IX, p. 632 and J. Chem. Soc., Chem.Commun. (1984) 874), and by imine oxidation withN-methylhydroxylamine-O-sulfonic acid (Synthesis (1977) 318).

[0017] The organometallic complex optionally includes one or moreadditional polymerization-stable, anionic ligands. Examples includesubstituted and unsubstituted cyclopentadienyl, fluorenyl, and indenyl,or the like, such as those described in U.S. Pat. Nos. 4,791,180 and4,752,597, the teachings of which are incorporated herein by reference Apreferred group of polymerization-stable ligands are heteroatomicligands such as boraaryl, pyrrolyl, indolyl, quinolinyl, pyridinyl, andazaborolinyl as described in U.S. Pat. Nos. 5,554,775, 5,539,124,5,637,660, and 5,902,866, the teachings of which are incorporated hereinby reference. Suitable polymerization-stable ligands includeindenoindolyl anions such as those described in PCT publication WO99/24446. The organometallic complex also usually includes one or morelabile ligands such as halides, alkyls, alkaryls, aryls, dialkylaminos,or the like. Particularly preferred are halides, alkyls, and alkaryls(e.g., chloride, methyl, benzyl).

[0018] The chelating N-oxide ligands and/or polymerization-stableligands can be bridged. For instance, a —CH₂—, —CH₂CH₂—, or (CH₃)₂Sibridge can be used to link two chelating N-oxide ligands or an N-oxideligand and a polymerization-stable ligand. Groups that can be used tobridge the ligands include, for example, methylene, ethylene,1,2-phenylene, and dialkyl silyls. Normally, only a single bridge isincluded. Bridging changes the geometry around the transition orlanthanide metal and can improve catalyst activity and other propertiessuch as comonomer incorporation.

[0019] Exemplary organometallic complexes:

[0020] zirconium (2-oxypyridine N-oxide) trichloride,

[0021] titanium (2-oxypyridine N-oxide) trimethyl,

[0022] zirconium bis(8-oxyquinoline N-oxide) dichloride,

[0023] zirconium bis(2-(N′-methylamido)pyridine N-oxide) dichloride,

[0024] hafnium (N-(2-oxyethyl)-N,N-di-n-butylamine N-oxide) trichloride

[0025] zirconium (2-oxymethyl-N-methylpiperidine N-oxide) trichloride

[0026] titanium (2-oxypyridine N-oxide) tribenzyl

[0027] zirconium (1-methylborabenzene)(2-oxypyridine N-oxide) dichloride

[0028] zirconium (cyclopentadienyl)(2-oxypyridine N-oxide) dimethyl andthe like.

[0029] The catalysts include an activator. Suitable activators ionizethe organometallic complex to produce an active olefin polymerizationcatalyst. Suitable activators are well known in the art. Examplesinclude alumoxanes (methyl alumoxane (MAO), PMAO, ethyl alumoxane,diisobutyl alumoxane), alkylaluminum compounds (triethylaluminum,diethyl aluminum chloride, trimethylaluminum, triisobutyl aluminum), andthe like. Suitable activators include acid salts that containnon-nucleophilic anions. These compounds generally consist of bulkyligands attached to boron or aluminum. Examples include lithiumtetrakis(pentafluorophenyl)borate, lithiumtetrakis(pentafluorophenyl)aluminate, aniliniumtetrakis(pentafluorophenyl)borate, and the like. Suitable activatorsalso include organoboranes, which include boron and one or more alkyl,aryl, or aralkyl groups. Suitable activators include substituted andunsubstituted trialkyl and triarylboranes such astris(pentafluorophenyl)borane, triphenylborane, tri-n-octylborane, andthe like. These and other suitable boron-containing activators aredescribed in U.S. Pat. Nos. 5,153,157, 5,198,401, and 5,241,025, theteachings of which are incorporated herein by reference.

[0030] The amount of activator needed relative to the amount oforganometallic complex depends on many factors, including the nature ofthe complex and activator, the desired reaction rate, the kind ofpolyolefin product, the reaction conditions, and other factors.Generally, however, when the activator is an alumoxane or an alkylaluminum compound, the amount used will be within the range of about0.01 to about 5000 moles, preferably from about 0.1 to about 500 moles,of aluminum per mole of M. When the activator is an organoborane or anionic borate or aluminate, the amount used will be within the range ofabout 0.01 to about 5000 moles, preferably from about 0.1 to about 500moles, of activator per mole of M.

[0031] If desired, a catalyst support such as silica or alumina can beused. However, the use of a support is generally not necessary forpracticing the process of the invention.

[0032] The invention includes methods for making the organometalliccomplex. One method comprises deprotonating a chelating N-oxide anionprecursor with at least one equivalent of a potent base such as lithiumdiisopropylamide, n-butyllithium, sodium hydride, a Grignard reagent, orthe like. The resulting anion is reacted with a Group 3 to 10 transitionor lanthanide metal source to produce an organometallic complex. Thecomplex comprises the metal, M, and at least one chelating N-oxideligand that is bonded to the metal. Any convenient source of the Group 3to 10 transition or lanthanide metal can be used. Usually, the source isa complex that contains one or more labile ligands that are easilydisplaced by the N-oxide anion. Examples are halides (e.g., TiCl₄,ZrCl₄), alkoxides, amides, and the like. The metal source canincorporate one or more of the polymerization-stable anionic ligandsdescribed earlier. The organometallic complex can be used “as is.”Often,however, the complex is converted to an alkyl derivative by treating itwith an alkylating agent such as methyl lithium. The alkylated complexesare more suitable for use with certain activators (e.g., ionic borates).

[0033] The N-oxide anion is preferably generated at low temperature (0°C. to −100° C.), preferably in an inert solvent (e.g., a hydrocarbon oran ether). The anion is then usually added to a solution of thetransition or lanthanide metal source at low to room temperature. Afterthe reaction is complete, by-products and solvents are removed to givethe desired transition metal complex.

[0034] Some N-oxide ligands are nucleophilic enough to form suitablecomplexes without deprotonation. Much will depend upon the specifictransition metal used, the other ligands, the reaction conditions usedto make the complex, and other factors. In one suitable approach, achelating N-oxide compound that contains a labile hydrogen (e.g.,2-hydroxypyridine N-oxide) reacts directly with an alkyl-substitutedtransition metal complex (e.g., tetrabenzylzirconium) or with a metalhalide complex (e.g., cyclopentadienylzirconium trichloride) in thepresence of an acid scavenger such as triethylamine.

[0035] The catalysts of the invention are particularly valuable forpolymerizing olefins. Preferred olefins are ethylene and C₃-C₂₀α-olefins such as propylene, 1-butene, 1-hexene, 1-octene, and the like.Mixtures of olefins can be used. Ethylene and mixtures of ethylene withC₃-C₁₀ α-olefins are especially preferred.

[0036] Many types of olefin polymerization processes can be used.Preferably, the process is practiced in the liquid phase, which caninclude slurry, solution, suspension, or bulk processes, or acombination of these. High-pressure fluid phase or gas phase techniquescan also be used. The process of the invention is particularly valuablefor solution and slurry processes. Suitable methods for polymerizingolefins using the catalysts of the invention are described, for example,in U.S. Pat. Nos. 5,902,866, 5,637,659, and 5,539,124, the teachings ofwhich are incorporated herein by reference.

[0037] The olefin polymerizations can be performed over a widetemperature range, such as about −30° C. to about 280° C. A morepreferred range is from about 30° C. to about 180° C.; most preferred isthe range from about 60° C. to about 100° C. Olefin partial pressuresnormally range from about 15 psia to about 50,000 psia. More preferredis the range from about 15 psia to about 1000 psia.

[0038] Catalyst concentrations used for the olefin polymerization dependon many factors. Preferably, however, the concentration ranges fromabout 0.01 micromoles per liter to about 100 micromoles per literPolymerization times depend on the type of process, the catalystconcentration, and other factors. Generally, polymerizations arecomplete within several seconds to several hours.

[0039] The following examples merely illustrate the invention. Thoseskilled in the art will recognize many variations that are within thespirit of the invention and scope of the claims.

EXAMPLE 1 Preparation of Zirconium (2-oxypyridine N-oxide) Trichloride

[0040] Commercially available 2-hydroxypyridine N-oxide (1.0 g, 9.0mmol) in diethyl ether (25 mL) is deprotonated by careful addition ofn-butyllithium (6.25 mL of 1.6 M solution in hexanes, 10 mmol) at −78°C. The resulting anion is separated from excess salts by filtration invacuo.

[0041] The pyridinoxy anion solution is added by cannula to a stirredslurry of zirconium tetrachloride (2.05 g, 8.8 mmol) in diethyl ether(25 mL) at −78° C. The reaction mixture is stirred and allowed to warmto room temperature. Volatiles are removed in vacuo. The residue isextracted with toluene to give a solution of the organometallic complex.This solution can be used “as is” for polymerizing olefins. The expectedproduct is zirconium (2-oxypyridine N-oxide) trichloride.

[0042] Additional evidence for the suitability of chelating N-oxides asligands for single-site catalysts comes from molecular modeling studies.For example, using molecular orbital calculations at the PM3tm (Spartansoftware distributed by Wavefunction, Inc.), we found that zirconoceniumactive sites based on the anion derived from 2-hydroxypyridine N-oxidehave calculated reactivity indices (e.g., hardness and electrophilicity)that are remarkably similar to the values calculated for traditionalligands based on cyclopentadienyl anions. The model calculations suggestthat the electronic and steric environments of certain chelatingN-oxides make them an excellent choice as ligands for single-sitecatalysts.

[0043] The preceding examples are meant only as illustrations. Thefollowing claims define the invention.

I claim:
 1. A catalyst which comprises: (a) an activator; and (b) anorganometallic complex comprising a Group 3 to 10 transition orlanthanide metal, M, and at least one chelating N-oxide ligand that isbonded to M.
 2. The catalyst of claim 1 wherein the activator isselected from the group consisting of alumoxanes, alkylaluminumcompounds, organoboranes, ionic borates, and ionic aluminates.
 3. Thecatalyst of claim 1 comprising a Group 4 transition metal.
 4. Thecatalyst of claim 1 further comprising a substituted or unsubstitutedcyclopentadienyl, indenyl, or fluorenyl group.
 5. The catalyst of claim1 further comprising a polymerization-stable, anionic ligand selectedfrom the group consisting of boraaryl, pyrrolyl, indolyl, quinolinyl,pyridinyl, indenoindolyl, and azaborolinyl.
 6. The catalyst of claim 1wherein the chelating N-oxide ligand is bridged to another ligand. 7.The catalyst of claim 1 wherein the chelating N-oxide ligand is selectedfrom the group consisting of N-substituted imidazole N-oxides, pyridineN-oxides, quinoline N-oxides, and lutidine N-oxides that have anelectron-donating group in the ortho position.
 8. The catalyst of claim7 wherein the chelating N-oxide ligand is an oxy anion derived from2-hydroxypyridine N-oxide.
 9. The catalyst of claim 7 wherein thechelating N-oxide ligand is an oxy anion derived from 8-hydroxyquinolineN-oxide.
 10. The catalyst of claim 1 wherein the chelating N-oxideligand is an azoxybenzene or nitrone having an electron donor groupwithin 5 atoms of the N-oxide oxygen.
 11. A process which comprisespolymerizing an olefin in the presence of the catalyst of claim
 1. 12. Aprocess which comprises copolymerizing ethylene with a C₃-C₁₀ a-olefinin the presence of the catalyst of claim
 1. 13. A method which comprisesdeprotonating a chelating N-oxide precursor and reacting the resultinganion with a Group 3 to 10 transition or lanthanide metal source toproduce an organometallic complex comprising the metal, M, and at leastone chelating N-oxide ligand that is bonded to M.
 14. A method whichcomprises directly reacting a chelating N-oxide compound that has alabile hydrogen with an alkyl- or halide-substituted Group 3 to 10transition or lanthanide metal source, optionally in the presence of anacid scavenger, to produce an organometallic complex comprising themetal, M, and at least one chelating N-oxide ligand that is bonded to M.